Electrolyte, which is filling the interface between a semiconductor layer and a counter electrode in the dye-sensitized solar cell (DSSC), is usually a solution containing a I–/I3 – redox system. The task of the electrolyte is to transfer the electron to the oxidized dye, so that it returns to its basic state. The first liquid electrolyte used in the dye-sensitized solar cells was the LiI/I2. Currently, the most common and most effective electrolyte is the liquid electrolyte based on redox compound iodide/triiodide containing various organic solvents such as: acetonitrile, ethylene carbonate, 3-methoxy propionitrile and N-methylpyrrolidone [1, 2, 3, 4]. In the DSSC ionic liquids are also being used, i.e. liquids composed only of ions. Nowadays, the mentioned ionic liquid is understood as a salt in liquid form having a melting temperature not exceeding 100 °C. The disadvantage of liquid electrolyte is its ability to evaporate and leak. This results in a significant reduction in the life time and efficiency of dye-sensitized solar cells. There was an intensification of research into the replacement of liquid electrolyte. One solution is to use a material with ionic conductivity (so called Hole Transport Material – HTM). The first studies concerned inorganic materials such as CuI, CuBr, or CuSCN. However, despite the good conductivity of these materials (10–2S/cm), the stability of the solar cells prepared with them, is very poor [4, 5, 6]. Currently, the search for the successor of liquid electrolyte is being intensified towards the development of ionic conductive organic materials. Examples of use of various types of ion-conductive materials and their effect on DSSCs are shown in Table 1 [7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19].
The research presented in this paper focuses on the appropriate doping of the MEH-PPV polymeric material by adding potassium iodide to improve its electrical conductivity and the development of thin film deposition technology for use in dye-sensitized solar cells.
2 Materials and methodology
Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene], anhydrous potassium iodide (KI) and other chemicals were supplied by Sigma Aldrich. MEH-PPV is a conductive polymer with a width energy band gap equal 2.7 eV and red color, often used for the production of OLEDs with bright light. MEH-PPV is a derivative of PPV, which due to its asymmetric side chains is highly soluble in most organic solvents. N-Methylpyrrolidine (NMP) was used to dissolve the polymer and salt, and the liquid solutions were spin-coated to form polyelectrolyte thin films. Scanning Electron Microscopic (SEM) images were taken with a Zeiss Supra 35. Qualitative studies of chemical composition were also performed using the Energy Dispersive Spectrometer (EDS). The resistance of the prepared samples was measured using a Keithley meter. The meter was connected to a specialized adapter for testing thin films. Silver contacts connected to external electrodes was applied on the edge of the layer to a better connection and more accurate measurement. Resistance measurements were made using constant voltage measuring in real time of the running current. In order to test the suitability of developed materials for application in the construction of photovoltaic cells, a series of dye-sensitized solar cells ITO/TiO2/dye/active layer/Al were prepared. The active layer is made from pure MEH-PPV and doped with potassium iodide. Current-voltage characteristics of dye-sensitized solar cells, with polymer material, were performed at Standard Test Conditions (irradiance intensity 1000 W/m2, temperature 25°C, spectrum AM1.5), using PV Test Solutions Tadeusz Żdanowicz Solar Cell I-V Tracer System with solar simulator and Keithley 2400 source meter. The intensity of incident light was calibrated with silicon reference cell equipped with KG5 filter certified by National Renewable Energy Laboratory NREL.
3 Results and discussion
The surface of the MEH-PPV polymer layer dissolved in a NMP solvent deposited on a glass substrate was made in SEM at 50 000 magnification (Figure 1a). Only with such large magnification uniform structure had been recorded. There are no visible cracks or any precipitates. The SEM image of the surface of MEH-PPV polymer layer dissolved in NMP solvent with 5% KI deposited on glass substrate, was made for comparison at identical magnifications to that of pure polymer material (Figure 1b). The influence of potassium iodide participation on the surface structure of the polymer in the form of KI precipitates of up to about 100 nm was recorded. Further doping did not significantly affect the surface morphology of MEH-PPV. The occurrence of precipitates slightly above 100 nm (Figure 1c). With participation of the 15% of KI, large crystals of potassium iodide were recorded (even above 2μm). In addition, with the present precipitates, regular agglomerates of polymeric material with characteristic oval shapes had appeared (Figure 1d).
There was also provided a qualitative analysis by using energy dispersive spectrometry. Documented spectrum with reflections typical for carbon (0.277 keV) and oxygen (0.525 keV), which is provided from the MEH-PPV polymer layer and the carbonate (1.067 keV), silicon (1.739 keV) and calcium (3.689 keV) derived from the glass substrate (Figure 2a). The EDS spectrum of the MEH-PPV polymer dissolved in NMP solvent with 5% KI deposited on a glass substrate, additionally shows reflects from potassium (3.311 and 3.588 keV) and iodine (3.936 and 4.219 keV) (Figure 2b). Distribution maps of elements detected in the analyzed area of the MEH-PPV were made (Figures 3a-f). Uniform distribution of potassium iodide in the structure of the polymer MEH-PPV was recorded.
The resistance of the prepared samples was measured using a Keithley meter. The meter was connected to a specialized adapter for testing thin films. Measurement of volume resistance of the MEH-PPV polymer layer was measured on a glass substrate with ITO electrodes. Silver contacts connected to external electrodes was applied on the edge of the layer to a better connection and more accurate measurement. Resistance measurements were made using a constant voltage measuring in real time the current running. The average value of the measured resistance was equal 6.26·108 Ω (Table 2). The conductivity of the non-doped MEH-PPV layer was equal 2.47·10–11S·cm–1. The additive 5% potassium iodide reduced the resistance to 2.33·106 Ω. The conductivity of the doped polymer increased to 5.86·10–9S·cm–1, which is two orders of magnitude. The layer with 10% KI was characterized by a average value of measured resistance equal 3.45·105Ω. Conductivity increased to values of 4.68·10–8S·cm–1. The addition of 15% potassium iodide resulted in a significant decrease in the measured resistance and its average value was 5.50·103 Ω. The conductivity of the thin layer of the polymer PVDF-HFP with the additive of 15% KI was 2.69·10–6 S·cm–1.
Current voltage characteristics were measured (Figure 4) from which the open circuit voltage, short circuit current, fill factor, maximum power and efficiency were calculated (Table 3). The software used to calculate the solar cell parameters takes into account the intensity of the intensity of solar radiation incident on the solar cell and solar cell surface. The following mathematical formula was used:
where: Pin – intensity of solar radiation incident on the solar cell; A0 – solar cell surface.
A solar cell containing a layer of non-doped polymer MEH-PPV in its structure is characterized by a maximum power of 1.2 mW and an efficiency of 2.02%. The addition of 5% potassium iodide influenced especially on the increase of fill factor to 0.56. Thanks to that, the maximum power of the solar cell is equal to 1.3 mW and the efficiency 2.04%. For a solar cell with a MEH-PPV and an additive of 10% KI the maximum power was 1.6 mW, while the efficiency 2.74%. It was not decided to make a solar cell with a MEH-PPV layer containing 15% KI because large KI precipitation caused its instability, despite its conductivity increased with higher content of potassium iodide. The different concentration of KI was influenced on resistivity of the MEH-PPV thin film and on efficiency of the prepared solar cell (Figure 5).
Doping the MEH-PPV polymeric material with potassium iodide generates a large number of charge carriers causing an increase in its electrical conductivity. After adding 10% KI to MEH-PPV, its conductivity is increased by 3 orders of magnitude (from 2.47·10–11 to 4.68·10–8), and after adding 15% KI by 5 orders of magnitude (from 2.47·10–11 to 2.69·10–6). On the basis of the surface morphology, it was determined that the polyelectrolyte based on the MEH-PPV may comprise a maximum of 10% potassium iodide. Above this content, there are present large salt crystals, causing discontinuity of the thin layer. Of all the newly developed solar cells, the highest efficiency of 2.64% has a cell with a MEH-PPV polymer doped with 10% of potassium iodide.
The publication was co-financed by the statutory grant of the Faculty of Mechanical Engineering of the Silesian University of Technology in 2017.
Wang M., Greatzel C., Zakeeruddin S.M., Greatzel M., Recent developments in redox electrolytes for dye-sensitized solar cells, Energ. Environ. Sci., 2012, 5, 9394-9404. CrossrefWeb of ScienceGoogle Scholar
Fischer A., Pettersson H., Hagfeldt A., Boschloo G., Kloo L., Gorlov M., Crystal formation involving 1-methylbenzimidazole in iodide/triiodide electrolytes for dye-sensitized solar cells, Sol. Energ. Mat. Sol. C, 2007, 91/12, 1062-1065. Web of ScienceGoogle Scholar
Bay L., West K., Winther-Jensen B., Jacobsen T., Electrochemical reaction rates in a dye-sensitised solar cell the iodide/tri-iodide redox system, Sol. Energ. Mat. Sol. C, 2006, 90/3, 341-351. Google Scholar
Perera V.P.S., Tennakone K., Recombination processes in dye-sensitized solid-state solar cells with CuI as the hole collector, Sol. Energ. Mat. Sol. C, 2003, 79/2, 249-255. Google Scholar
Rusop M., Shirata T., Sirimanne P.M., Soga T., Jimbo T., Umeno M., Study on the properties and charge generation in dye-sensitized n-TiO2|dye|p-CuI solid state photovoltaic solar cells, Appl. Surf. Sci., 2006, 252/20, 7389-7396. Google Scholar
Gratzel M., Dye-sensitized solar cells, J. Photoch. Photobio. C, 2003, 4/2, 145-153. Google Scholar
Kokorin A., Ionic Liquids: Theory, properties, new approaches, InTech, 2011 Google Scholar
Hsu C.Y., Chung Chen Y., Yeh-Yung R., Kuo-Chuan H., Jiann T., Solidstate dye-sensitized solar cells based on spirofluorene (spiro-OMeTAD) and arylamines as hole transporting materials, Phys. Chem. Chem. Phys., 2012, 14, 14099-14109. CrossrefGoogle Scholar
Ren Y., Zhang Z., Fang S., Yang M., Cai S., Application of PEO based gel network polymer electrolytes in dye-sensitized photoelectrochemical cells, Sol. Energ. Mater., 2002, 71/2, 253-259. Google Scholar
Wang P., Zakeeruddin S.M., Exnar I., Grätzel M., High efficiency dye-sensitized nanocrystalline solar cells based on ionic liquid polymer gel electrolyte, Chem. Commun., 2002, 24, 2972-297. Google Scholar
Kim J.H., Kang M.S., Kim Y.J., Won J., Park N.G., Kang Y.S., Dye-sensitized nanocrystalline solar cells based on composite polymer electrolytes containing fumed silica nanoparticles, Chem. Commun., 2004, 14, 1662-1663. Google Scholar
Kang M.S., Kim J.H., Kim Y.J., Won J., Park N.G., Kang Y.S., Dye-sensitized solar cells based on composite solid polymer electrolytes, Chem. Commun., 2005, 7, 889-891. Google Scholar
Lee K.M., Suryanarayanan V., Ho K.C., A photo-physical and electrochemical impedance spectroscopy study on the quasi-solid state dye-sensitized solar cells based on poly (vinylidene fluoride-co-hexafluoropropylene), J. Power Sources, 2008, 185/2, 1605-1612. Web of ScienceGoogle Scholar
Wu J., Hao S., Lan Z., Lin J., Huang M., Huang Y., Sato T., An all-solid-state dye-sensitized solar cell-based poly (N-alkyl-4-vinyl-pyridine iodide) electrolyte with efficiency of 5.64%, J. Am. Chem. Soc., 2008, 130/35, 11568-11569. Web of ScienceGoogle Scholar
Yang H., Huang M., Wu J., Lan Z., Hao S., Lin J., The polymer gel electrolyte based on poly (methyl methacrylate) and its application in quasi-solid-state dye-sensitized solar cells, Mater. Chem. Phys., 2008, 110/1, 38-42. Web of ScienceGoogle Scholar
Bella F., Ozzello E.D., Sacco A., Bianco S., Bongiovanni R., Polymer electrolytes for dye-sensitized solar cells prepared by photopolymerization of PEG-based oligomers, Int. J. Hydrogen Energ., 2014, 39/6, 3036-3045. Web of ScienceGoogle Scholar
Bella F., Mobarak N.N., Jumaah F.N., Ahmad A., From seaweeds to biopolymeric electrolytes for third generation solar cells: an intriguing approach, Electrochim Acta, 2015, 151, 306-31. CrossrefWeb of ScienceGoogle Scholar
Darvishzadeh P., Redzwan G., Ahmadi R., Gorji N.E., Modeling the degradation/recovery of short-circuit current density in perovskite and thin film photovoltaics, Org. Electron, 2017, 43, 247-252. CrossrefWeb of ScienceGoogle Scholar
About the article
Published Online: 2017-12-29
Citation Information: Open Physics, Volume 15, Issue 1, Pages 1022–1027, ISSN (Online) 2391-5471, DOI: https://doi.org/10.1515/phys-2017-0127.
© 2017 Magdalena M. Szindler et al.. This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 License. BY-NC-ND 4.0